Wiktionary
constituent country
n. An administrative division of the United Kingdom, of which there are three: the kingdoms of England and Scotland, and the principality of Wales (Northern Ireland is a province).
Wikipedia
Constituent country
A constituent country is a country that makes up a part of a larger political or constitutional entity, usually a sovereign state. The term constituent country does not have any defined legal meaning, and is used simply to refer to a country which is a constituent part of something else.